40 



CYCLOSTOMATA 



The external opening of the hypophysial sac is single and median, 

 it forms the adult nostril, and it is perhaps chiefly owing to 

 this confluence of the hypophysis with the nasal pits that the 



Fig. 25. 



Head of Petromyzon fluviatilis, I... dissected : dorsal view. The brain and nerves have beeii 

 exposed entirely on the left and partially on the right side ; the eye, part of the skull, and 

 muscles ate retained on the right ; and the nasal capsule has been opened in front, c, choroid 

 plexus "u roof of 4th ventricle ; etc, connection between facial and glossopharyngeal and vagus 

 nerves ; cr, cranium ; er.t, anterior region of trabecula ; d.g, ganglion on dorsal spinal root ; c, 

 eye; h, habenular ganglia; /, labyrinth of ear exposed; LI, lateral-line nerve; m.o, medulla 

 oblongata : my, myotome ; n.a, neural arch cut through ; we, spinal cord ; n.ea, nasal capsule ; 

 istril ; o.a, anterior oblique muscle ; o.c, auditory capsule; .../, optic h>be ; o/./, olfactory 

 lobe; am, olfactory fold ; os, posterior oblique muscle; />, pineal eye; p.r, posterior rectus-; 

 ps, opening to hypophysial sac; r.a, anterior rectus; $.r, superior rectus; m, vagus epi- 

 branchial nerve lifted up above the spinals ; v.r, ventral spinal root, l-'.i the first nine cranial 

 nerves. 



latter have come to acquire the appearance of a single median 

 organ. 



The hypophysial sac of the Cyclostomes is unique in that it 



persists in the adult opening separately and dorsally to the 



